Membrane fouling is generally recognised as the outstanding problem in modern ultrafiltration. A full discussion of the problems can be found in "Fifteen Years of Ultrafiltration" by Michaels, A.S. in Ultrafiltration Membranes and Applications edited by A. R. Cooper (American Chemical Society Symposium, Washington, 9-14 Sept. 1979, Plenum Press, New York (1980); ISBN 0-306-40548-2) where it is stated:
"the problems of reduced throughput, capacity, increased power consumption, compromised separation capability, and reduced membrane service lifetime associated with macro-, solute- and colloid-fouling of ultrafiltration membranes have stubbornly resisted adequate solution despite ten years of engineering experience in pilot- and full-scale industrial situations."
According to Michaels, back-washing by reverse flow of permeate in hollow-fibre membrane modules, significantly aids unplugging of membrane pores and detachment of adhering deposits. However, there are only two specific examples of permeate back-washing described in this text and these concern filtration of town water and of electro-deposition paints emulsified in water.
As set forth at pages 109 to 127 of the above text, back-washing of hollow fibres with permeate is used where operating transmembrane pressures are only about one atmosphere so that particles are not driven hard into membrane pores during the filtering process. As indicated above, permeate back-washing has been used where the fouling species are in liquid paint emulsion droplets as these species do not wedge into the membrane pores as do solids. As the transmembrane flux is often only five to twenty liters per square meter per hour (L/m.sup.2 hr), the corresponding fluid velocity is only a few millimeters per hour and there is, therefore, no possibility of a high velocity cleaning action.
Permeate back-washing is, in essence, a recycling process and thus a sacrifice of production rate is only justified when the cleaning effect is significant. Some sticky natural wastes (such as brewing residues, starch, and egg) are not removed to any appreciable extent by permeate back-washing. Permeate back-washing is, by definition, a purely hydraulic flow through totally wetted pores of the ultrafiltration membrane. Hollow, porous fibre ultrafilters are preferred where back-wash cleaning is needed because of the structure of the hollow fibres.